[Fasta] include source code in dill

pkg/kernel/lib/binary/ast_to_binary.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.ast_to_binary;
 
 import '../ast.dart';
 import '../import_table.dart';
 import 'tag.dart';
 import 'dart:convert';
 import 'dart:typed_data';
@@ skipping 51 matching lines @@
     }
   }
 
   void writeMagicWord(int value) {
     writeByte((value >> 24) & 0xFF);
     writeByte((value >> 16) & 0xFF);
     writeByte((value >> 8) & 0xFF);
     writeByte(value & 0xFF);
   }
 
+  void writeUtf8Bytes(List<int> utf8Bytes) {
+    writeUInt30(utf8Bytes.length);
+    writeBytes(utf8Bytes);
+  }
+
   void writeStringTableEntry(String string) {
     List<int> utf8Bytes = const Utf8Encoder().convert(string);
     writeUInt30(utf8Bytes.length);
     writeBytes(utf8Bytes);
   }
 
   void writeStringTable(StringIndexer indexer) {
     writeUInt30(indexer.numberOfStrings);
     for (var entry in indexer.entries) {
       writeStringTableEntry(entry.value);
@@ skipping 82 matching lines @@
     _flush();
   }
 
   void writeUriToSource(Program program) {
     program.uriToSource.keys.forEach((uri) {
       _sourceUriIndexer.put(uri);
     });
     writeStringTable(_sourceUriIndexer);
     for (int i = 0; i < _sourceUriIndexer.entries.length; i++) {
       String uri = _sourceUriIndexer.entries[i].value;
-      Source source = program.uriToSource[uri] ?? new Source([], '');
-      String sourceCode = source.source;
-      writeStringTableEntry(sourceCode);
+      Source source =
+          program.uriToSource[uri] ?? new Source(<int>[], const <int>[]);
+      writeUtf8Bytes(source.source);
       List<int> lineStarts = source.lineStarts;
       writeUInt30(lineStarts.length);
       int previousLineStart = 0;
       lineStarts.forEach((lineStart) {
         writeUInt30(lineStart - previousLineStart);
         previousLineStart = lineStart;
       });
     }
   }
 
@@ skipping 1088 matching lines @@
   void flush() {
     _sink.add(_buffer.sublist(0, length));
     _buffer = new Uint8List(SIZE);
     length = 0;
   }
 
   void flushAndDestroy() {
     _sink.add(_buffer.sublist(0, length));
   }
 }